(A) Field of the Invention
The present invention relates to a composite material which carries zinc oxide particles, adhered thereto, (hereinafter referred to as "zinc oxide particle-adhered composite material") which shows various activities such as antibacterial, deodorizing, UV absorbing, photocatalytic, stain-proofing and purifying activities and a method for preparing the composite material. More specifically, the present invention relates to a zinc oxide particle-adhered composite material wherein zinc oxide particles are firmly adhered to the surface of a substrate without using any binder and exposed on the surface and exhibit antibacterial, deodorizing, UV absorbing, photocatalytic, stain-proofing and purifying activities as well as a method for preparing the composite material.
(B) Description of the Prior Art
It has been known that zinc oxide shows an antibacterial activity; such a deodorizing activity that it can selectively absorb sulfur-containing gases such as hydrogen sulfide and sulfur dioxide gas; a UV absorbing activity; a photocatalytic activity like titanium oxide; and stain-proofing and purifying activities. It has been reported that a photocatalytic substance such as zinc oxide shows its photocatalytic activity when light rays including ultraviolet rays irradiate the catalyst and the oxidation activity thereof is induced by the photocatalytic activity (generally referred to as "photocatalytic reaction") to thus decompose gases which give out a bad smell and to show its antibacterial activity.
Zinc oxide particles have often been fixed to a substrate for making the most use of the foregoing activities such as antibacterial, deodorizing, UV absorbing, photocatalytic, stain-proofing and/or purifying activities. As methods for fixing zinc oxide particles to the substrate, there have conventionally been proposed and used, for instance, a method which comprises kneading zinc oxide fine particles into a resin as a molding material to thus incorporate the particles into resin molded articles or fibers; or a method which comprises dispersing zinc oxide fine particles in paint and varnish to give dispersed zinc oxide particle-containing paint and varnish and applying the paint and varnish to a desired substrate, or applying a dispersion comprising zinc oxide particles and a binder to a substrate and then subjecting the coated substrate to a heat-treatment to thus firmly adhere the particles to the substrate (see, for instance, JP-A-5-156510).
If kneading zinc oxide particles into a resin or dispersing them in paint and varnish as described above, most of the zinc oxide particles are enclosed or embedded in resins or paint and varnish and are hardly exposed on the surface thereof. For this reason, the strong antibacterial and deodorizing activities of zinc oxide fine particles are not effectively used and when such paint and varnish is applied onto fibrous products, the fibrous products lose their flexibility and the paint and varnish cannot easily be applied uniformly to the products. As a means for eliminating this problem, there has been proposed a method which comprises kneading zinc oxide particles into a resin and then etching the surface of the resulting resin by, for instance, irradiation thereof with an ion beam or ozone to thus expose, on the surface, zinc oxide particles as much as possible (see, for instance, JP-A-1-156576). However, this method does not permit a substantial increase in the number of zinc oxide particles exposed on the surface and a new problem arises, i.e., such an etching treatment would rather make the resin or fibers brittle.
In case where a dispersion is applied onto a substrate (the method disclosed in JP-A-5-156510), the coated layer must be subjected to a heat-treatment in order to evaporate the dispersion medium and to ensure the adhesion of particles to the substrate by the use of a binder. Moreover, the activity of zinc oxide particles per se and the effects which may be expected by the incorporation of zinc oxide particles would be deteriorated or impaired because of the adhesion of the binder onto the surface of the particles. Accordingly, the antibacterial effect thereof is low and almost equivalent to that observed when they are kneaded into resins. Further the resins and binders would be deteriorated due to the choking effect of zinc oxide.
Also known is a method which comprises the steps of covering a substrate with zinc hydroxide and then decomposing, with heating, the layer of zinc hydroxide to give a coating layer of zinc oxide, but in this case, the substrate should be heated to a high temperature on the order of not less than 150.degree. C. and this may, in turn, be accompanied by deformation, change of properties or melting of the substrate depending on the material of the substrate. Moreover, the coated film has a tendency to be easily peeled off during heating and accordingly, it is difficult to obtain a body carrying zinc oxide firmly adhered thereto.
The thermal spraying technique may be used as the method for adhering zinc oxide particles to the surface of a substrate, but the particle size of the zinc oxide particles formed by this method is not less than several micrometers (.mu.m) and these particles are welded together. Therefore, if molten zinc oxide particles are adhered to, for instance, a fibrous substance as the substrate, the substrate loses its softness and the touch and is not practically acceptable. Zinc oxide may likewise be adhered to a substrate by the sputtering and vapor deposition techniques, but these techniques simply permit the coating of the outermost layer of a substrate and cannot uniformly cover the entire surface of, for instance, fibrous products (including the surface of internal fibers thereof) with zinc oxide particles.
Organic and inorganic chemicals other than zinc oxide have been used as antibacterial and/or deodorizing agents and some organic drugs have sometimes been applied to the surface of a substrate. However, zinc oxide is desirably used as a material which has not only safety and durability, but also antibacterial and deodorizing activities.
In addition, JP-A-4-164813 discloses a method for preparing zinc oxide fine particles comprising the steps of dropwise adding an aqueous alkaline solution to an aqueous solution of a zinc salt at a temperature of not less than 60.degree. C. and precipitating zinc oxide particles at a final pH of not less than 9 and JP-A-4-164814 discloses a method for preparing zinc oxide fine particles comprising the steps of dropwise adding an aqueous solution of a zinc salt to an aqueous alkaline solution at a temperature of not less than 60.degree. C. and precipitating zinc oxide particles at a final pH of not less than 9, but these patents do not disclose a zinc oxide fine particle-adhered composite material wherein the zinc oxide fine particles are firmly bonded to the surface of a substrate without using any binder and are sufficiently exposed on the surface as well as a method for preparing the composite material.